Battery Calculator 2025: How to Size Your Home System and Save
Thinking About a Home Battery in 2025? Here’s How to Calculate the Right Size and ROI
With battery rebates rolling out across Australia in 2025, homeowners everywhere are asking the same questions:
- How big should my battery be?
- Will it power my home through a blackout?
- Will it pay for itself?
In this guide, we walk through simple and advanced battery calculators to help you understand the ideal size for your home battery, how to calculate your return on investment, and what you can expect from modern solar and battery proposals.
Why Battery Size Matters
You don’t want a battery that’s too big—it may never fully charge from your solar system. But you also don’t want one too small that runs out of power before dinner. The key is to match your battery size to your solar output and your energy use across the day and night.
Quick Battery Sizing Rule
Most Aussie homes consume around 20kWh of electricity daily. That typically breaks down to:
- 5kWh during the day
- 15kWh at night
A simple rule: divide your daily consumption (20kWh) by 4. This gives you a recommended 5kW solar system. It will generate about 20kWh/day (5kW x 4 sun hours):
- 5kWh goes to your daytime usage
- 15kWh charges the battery for use at night
Using Advanced Battery Calculators
For larger homes or higher usage, you might need to go deeper. Estimate your average daily consumption by checking your electricity bill and dividing total kWh by the number of days in the billing cycle. Then break that down into daytime and nighttime percentages (e.g. 25% day, 75% night).
If your average daily use is 40kWh, and 30kWh is at night, you’ll want:
- 10kWh for daytime offset
- 30kWh battery for night
- At least a 10kW solar system (ideally more to cover seasonal variations)
Planning for Backup Power
Batteries are also valued for backup in blackouts. To calculate how much storage you need:
- Light (100W x 5h) = 0.5kWh
- Fridge (2kW x 5h) = 10kWh
Add up all devices you want on backup. If your total comes to 10.5kWh, you’ll want a battery that’s at least 12kWh so you’re not fully draining it every time.
Do You Want to Trade on a VPP?
Virtual Power Plants (VPPs) let you sell excess stored energy during peak times. If you allocate 20% of your 10kWh battery (2kWh) to the VPP, you’ll have 8kWh for personal use. Retailers control this based on forecasts and may offer revenue in return.
Reading a Solar + Battery Proposal 10kW solar + 10kWh battery
- Estimated generation: ~56kWh/day in summer, ~26kWh/day in winter
- This averages ~40kWh/day annually.
Expected Annual Savings
Savings come from:
- Solar self-consumption during the day
- Battery offset during peak evening rates
- VPP or grid services like FCAS
Example:
- Day usage offset: 15kWh x $0.50 = $7.50
- Battery offset: 10kWh x $0.50 = $5.00
- Grid feed-in: 15kWh x $0.05 = $0.75
- Total: $13.25/day x 365 = ~$4,836/year
Adjusted for real-world pricing (~$0.35/kWh), total savings are closer to $3,200/year.
What About ROI?
Let’s say your system costs $24,000 upfront. With rebates ($3,300 solar + $3,800 battery), you’ll pay ~$17,000 out of pocket.
- Savings: $3,200/year
- Payback: ~5.3 years
- IRR: 11%
Financing at $0 upfront? Monthly repayments can match your current energy bill, meaning no added expense.
Bonus: At the end of the term, the system adds resale value to your home.
Ready to Size Your System?
Use our free battery calculator to:
- Find your ideal solar + battery size
- Estimate savings
Model backup and VPP capacity
And get 3 no-obligation quotes from trusted local installers now here
Investing in solar and batteries is no longer just about the environment—it’s a smart financial move.
